1. Field of the Invention
This invention relates to a method and apparatus for measuring, without contact, the emissivity of product which can be in moving.
2. Description of the Prior Art
Such a measurement is important because in any temperature determination by means of an optical pyrometer operating in a given spectral band, the signal delivered by the pyrometer can only be converted into temperature units if the emissivity of the product in this spectral band is known, as the signal delivered by the pyrometer is proportional to the emissivity. Because of this, it is apparent that the problem of making an exact continuous measurement of a temperature by means of an optical pyrometer is all the more difficult because the emissivity of the product can vary with time, owing to changes in its surface state, oxidation, the quality and the nature of the product, etc.
There are several different methods for measuring the emissivity of a product, but they generally have the drawbacks of extreme slowness, of being able to be operated only in the laboratory, of requiring destruction-sampling of the product, and often of requiring complementary processing of the signal received in order to obtain the required emissivity. One of these well-known methods is operated as follows.
An almost totally reflecting enclosure placed in contact with a surface constitutes a black body about said surface. The black body thus constituted about the said surface has the effect of making the value of the effective emissivity of said surface very close to unity because of the multiple reflections produced inside the reflecting enclosure. Let V.sub.1 be the signal provided by a pyrometer situated inside the said enclosure and operating within a spectral band .lambda..
An almost totally absorbing enclosure placed in contact in the same position with the same surface allows no reflection. Let V.sub.2 be the signal provided by a pyrometer operating within the same spectral band .lambda. and situated inside such an absorbing enclosure.
The emissivity of the said surface, when measured within the spectral band .lambda. considered, is a function of the ratio V.sub.2 /V.sub.1 in accordance with a given known law. In addition, the signal delivered by the pyrometer situated in the reflecting enclosure enables the actual temperature of the product to be obtained, independently of its own emissivity.